Gate valves used for control of petroleum production are operated at high pressure and temperature. The well shut in pressure can be above 103,5 MPa, the contact pressure between valve parts is even higher, whilst the temperature can be above 177° C. In operation the valve can be in one position for long periods of time, such as in position open for petroleum production. According to standards, such as Norsok, the function of the valve is to be tested periodically, such as every third month.
Unfortunately the valves have an inherent friction problem. More specifically, the valve parts have a tendency of sticking, adhering or cold welding to each other, resulting in high friction. This is also termed stick slipping and galling. In order to overcome the friction problem, the actuators, umbilicals and other associated equipment must be dimensioned in order to operate the valve in a state of enhanced friction. Accordingly, actuators, umbilicals and other associated equipment, in addition to the valve itself or parts thereof, must be up to several times larger, stronger or expensive than required compared to the friction level at its lowest.
An approach that has been used for some time in order to overcome the problems is to apply coating on sealing faces of the valve. More specifically, coating of tungsten carbide is used. Currently, coating of tungsten carbide on surfaces subject to sliding is an industry standard for gate valves for control of petroleum production, particularly subsea petroleum production control valves. Despite the tungsten carbide coating, the sticking together of valve parts is still a severe problem. The effect of the sticking or friction problem is not only enhanced friction but also stick slipping or uneven movement of the valve gate during operation as compared to a smooth, even, continuous movement.
Friction tests reveal that the friction problem should have been solved by the coatings according to the industry standard. However, practical experience reveal that the friction sometimes increases to an enhanced level over time, from a lower start up level of friction, which is very surprising in view of the friction test results. Even though the friction problem has been encountered for many years, it has not yet been possible to mitigate.
US 20070163655 A1 describes a gate valve for use with well fluids, where the surfaces meant to slide against each other are coated with a hard carbide surface and a top layer of diamond like carbon (DLC). This document does not describe embodiments where the valve is used on subsea production equipment. Neither does the document mention the importance of hydrogen in the DLC.
US 2004118455 A1 also describes a valve where carbide and DLC are used on the sliding surfaces.
Luoa, S. Y. et al.: “A study of the water behavior of diamond like carbon films under the dry reciprocating sliding contact”, Wear, 2001, vol. 249, pp. 800-807, describes that the coefficient of friction between surfaces treated with DLC and other surfaces, are reduced when the surfaces has passed through a certain number of sliding cycles.
The inventors have investigated the problem in depth in order to find further details on the problem, including when the problem occur and finding a solution to overcome the problem if possible. More specifically, the investigations revealed that larger surfaces appear sometimes to behave very different with respect to friction than smaller samples typically used for testing, even though the surfaces and coatings are identical, a large scale problem sometimes occur. More specifically, the investigations revealed that a severe friction problem can occur in a full scale real gate valve even though testing on smaller samples do not indicate any problem. This is surprising since the test procedures have been determined by experts in the field. Furthermore, valve fluid without oil contents, such as dry gas or water without oil, are found to enhance the problem in general, resulting in increased friction over time. Eventually, highly polished surfaces seem to increase the problem with respect to the unexpected size effect and the unexpected fluid type effect.
When the friction problem occurs, it will in general increase with time or operation cycles in a real full scale gate valve. The problem is still not quantified in great detail and the mechanisms resulting in the problems are not yet understood in great detail. But in addition to what is mentioned above the problems have been found to in general increase with increasing level of friction, increasing sliding distance, increasing contact pressure, increasing temperature, and particularly, the problems increase with time or operation cycles. The prevailing mode of friction is probably a dry or mixed type of friction. The problem is found to be very relevant for gate valves, particularly subsea gate valves, presumably due to large sealing surfaces, high pressures and long sliding distance.
The objective of the present invention is to reduce or eliminate the friction problem discussed above.
These objectives are obtained by a gate valve according to claim 1, a method of commissioning or operating a gate valve according to claim 5, and use of a diamond like carbon (DLC) top layer according to claim 8. Additional preferred embodiments are claimed in the dependent claims.